Conventional pressure-sensitive adhesives are hydrophobic in nature. As such, they adhere poorly to wet or damp surfaces and cannot generally be effectively removed by water, even using automated cleaning procedures which employ detergents, warm water, and/or mild caustics.
While water resistance of conventional pressure-sensitive adhesives is an obvious advantage to many enduse applications, there are certain applications where it is desired to have an adhesive adhere to a wet surface. For instance, in the pharmaceutical and beverage industries, it is frequently desirable to label bottles and other containers in-line immediately after washing and rinsing processes, while the containers are still surface-wet. In this application, conventional pressure-sensitive adhesives simply will not adhere satisfactorily.
For many applications it is desirable to use pressure-sensitive adhesives for permanent labeling of an object to provide sufficient tamperproofness so that labels cannot be removed intact while the label is still needed. Examples include price-marking labels which cannot be transferred by a customer in retail outlets, and product-identification labels used on recyclable containers. While conventional pressure-sensitive adhesives can meet the tamperproofness requirements, they are very difficult to remove when the function of tamperproofness is no longer required.
It would be desirable, therefore, to have a pressure-sensitive adhesive which is tamperproof under ordinary use conditions but which can be easily removed with water in a household and/or commercial cleaning process(es).
Several attempts have been made to develop water-soluble, pressure-sensitive adhesives, but the adhesives heretofore known have suffered from deficiencies in performance, which has retarded their utility.
Adhesives based on polyvinyl methyl ether were among the earliest water-soluble, pressure-sensitive adhesives. While they had fair adhesive properties and could be used for limited applications, several deficiencies restricted their utility. They did not, for instance, absorb water rapidly enough to adhere to a wet surface. In addition, they were insoluble in hot water, and could not be removed by commercial cleaning equipment. Also, their holding power under stress prohibited use on small-diameter cylindrical objects where the paper facestock used was conventional.
Partial esters such as polyvinylmethyl ether/maleic anhydride with nonionic surfactants of the nonyl phenol ethylene oxide adduct type have also been used. These adhesives were soluble in both hot and cold water for easy removability, but were so extremely humidity-sensitive that they could not be used for application to small items or objects because of low adhesion at low relative humidity and ready cohesive failure at high relative humidity.
Other water-soluble, pressure-sensitive adhesives have been developed which include compounds of polyvinyl pyrrolidone with water-soluble plasticizers, copolymers of acrylic acid and alkyl acrylates and others, but none has had the desired good pressure-sensitive adhesive properties, rapid adhesion to wet surface and easy removability in commercial equipment, and retention of the desired properties at high and low relative humidities.
A need therefore exists for an adhesive which has good adhesion to both wet and dry surfaces, whether cold or warm, whether polar or non-polar, with sufficient permanence to be regarded as tamperproof when combined with ordinary label papers; but yet easily removed, when desired, with warm or cold water or with detergents and alkalis used in commercial, or even domestic, cleaning operations. These objectives, as well as the objective of providing adhesives which resist degradation or deterioration under conditions of high and low relative humidity, are the subject of the instant invention.